Protein motions control enzyme catalysis through mechanisms that are incompletely understood. Here NMR <sup>13</sup>C relaxation dispersion experiments were used to monitor changes in side-chain motions that occur in response to activation by phosphorylation of the MAP kinase ERK2. NMR data for the methyl side chains on Ile, Leu, and Val residues showed changes in conformational exchange dynamics in the microsecond-to-millisecond time regime between the different activity states of ERK2. In inactive, unphosphorylated ERK2, localized conformational exchange was observed among methyl side chains, with little evidence for coupling between residues. Upon dual phosphorylation by MAP kinase kinase 1, the dynamics of assigned methyls in ERK2 were altered throughout the conserved kinase core, including many residues in the catalytic pocket. The majority of residues in active ERK2 fit to a single conformational exchange process, with k<sub>ex</sub> ≈ 300 s<sup>−1</sup> (k<sub>AB</sub> ≈ 240 s<sup>−1</sup>/k<sub>BA</sub> ≈ 60 s<sup>−1</sup>) and p<sub>A</sub>/p<sub>B</sub> ≈ 20%/80%, suggesting global domain motions involving interconversion between two states. A mutant of ERK2, engineered to enhance conformational mobility at the hinge region linking the N- and C-terminal domains, also induced two-state conformational exchange throughout the kinase core, with exchange properties of k<sub>ex</sub> ≈ 500 s<sup>−1</sup> (k<sub>AB</sub> ≈ 15 s<sup>−1</sup>/k<sub>BA</sub> ≈ 485 s<sup>−1</sup>) and p<sub>A</sub>/p<sub>B</sub> ≈ 97%/3%. Thus, phosphorylation and activation of ERK2 lead to a dramatic shift in conformational exchange dynamics, likely through release of constraints at the hinge.